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    <title>Document</title>
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    <script>
    /*  +,-,*,/,%   */
    /* 先将两边的值转为Number再计算 
    特殊：+运算列外
    */
   var a="12";
   var b=3;
   var c="12px";
   var d;
   console.log(a-b);
//    console.log(Number(a)-b)
    </script>
</body>
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